1. Field of the Invention
The present invention relates to novel electrical devices fabricated from polycrystalline silicon carbide (SiC) and methods for forming the same. More specifically, the present invention provides a method for fabricating polycrystalline silicon carbide (SiC) products infiltrated with SiC-containing preceramic precursor resins to substantially mask the deleterious effects of trace contaminants, typically nitrogen and aluminum, while reducing operative porosity and enhancing manufacturing ease.
2. Description of the Related Art
Prior to the advent of electric ignition controls, gas appliances and equipment were manually lit with a match or had continuously burning pilot lights. Although a few appliances still use these methods, the vast majority employ some form of spark or hot surface ignition.
Spark ignition systems are conventionally known to make use of an electronic control and an electrode located near a metal burner port that is electrically grounded. In operation, when there is a call for heat by a controlling thermostat or other means, a control opens a gas-supply valve and simultaneously provides a high voltage discharge (typically 20 kilovolts) to the electrode, which then “sparks” to the burner. This spark, in the presence of gas, ignites the burner.
In contrast to conventional spark systems, the controls used with SiC-based hot surface ignition operate differently. In operating a hot surface ignition system, when there is a call for heat, electrical power is applied to the igniter. Thereafter, after the igniter is allowed time to reach an operational temperature hot enough for ignition, a gas-supply valve is opened.
It should be noted that both types of ignition systems described (spark ignition and hot surface ignition) have safety features, which prevent continued gas flow in the absence of flame.
In 1988, minimum efficiency standards for most major gas appliances were instituted by the U.S. Department of Energy. These standards largely led to the demise of the standing pilot light for many appliance types and greatly increased the market potential for gas igniters. The positive market effect for SiC hot surface ignition surpassed that of electronic spark ignition based on a number of factors, including:                e. A recognition that the physical position of a poorly adjusted spark igniter electrode can shift, thereby reducing the spark's ability to light the burner. In contrast, due to its relatively large thermal mass within the gas-stream the hot surface igniter is much less position sensitive than a spark electrode and thus provides a more reliable source of ignition.        f. A recognition that the high voltage discharge inherent to spark systems can cause interference within the electromagnetic spectrum, specifically radio frequency interference in nearby electronic devices such as radios, TV's, computers, etc.        g. A recognition that the metal alloys used for spark electrodes erode and become pitted with use, reducing their effectiveness. In contrast the SiC materials used in conventional hot surface igniters withstands much higher temperatures than metal alloys.        h. A recognition that in most applications, a total system cost of using a SiC hot surface igniter is less than a comparable spark ignition system.        
These types of hot surface SiC igniters may have various physical shapes, for example, the SiC hot surface igniter shown in U.S. Pat. No. 3,875,477 to Fredriksson, et al., the contents of which are incorporated herein by reference. As referred to hereafter, the igniter noted in Fredriksson shall be understood to represent a thin-profile igniter having a first wide cross-section parallel to the plane shown in FIG. 1 in U.S. Pat. No. 3,875,477 and a second narrow cross-section perpendicular to the first cross-section.
As shown, this type of hot surface igniter provides a serpentine-form of electrical connection from two pole ends. A central region between the pole ends, having a reduced cross-section, is more resistive to electrical conduction, heats rapidly to allow hot surface ignition. While the present discussion, and methods and apparatus discussed, must be recognized to have broader application throughout the industry, this example form aids the general understanding without limiting the present application in any way to the form noted.
Yet despite the recognition provided to SiC hot surface igniters, certain detriments remain. It is now recognized that the density of the SiC hot surface igniter is critical in determining its electrical stability and mechanical strength. These factors (electrical stability and mechanical strength) are the two most important factors in the user-perceived quality and operating life of the igniter. Unfortunately, despite the above-recognition, in existing SiC igniter product examples, a typical 84% to 86% SiC density corresponds to a relatively high 16% to 14% interspermosing (meaning operable or flow-able interstecies between particles) porosity which is easily permeated by air and moisture, much like an open cell foam structure. Such detrimental porosity leads directly to excessive material oxidation at operational temperatures, mechanical stress, electrical property “aging” and resultant premature failure of the product.
Recently, in attempts to overcome the now-recognized detriments of SiC hot surface igniters several new types of ceramic hot surface igniters have emerged. These products (these grouped together and referred to as “nitride” igniters herein) are superior in mechanical strength but cannot match the service longevity and functional robustness of conventional SiC igniters. Specifically, nitride ceramics are limited in their detrimental high temperature capability in oxidizing environments as compared to silicon carbide (SiC) igniters.